With the aim of improving recording quality of ink-jet recording, application of a pretreatment liquid to a recording paper has been conducted by preparing the pretreatment liquid separately from an ink. For example, pretreatment liquids containing aggregating agents have been disclosed. By applying the pretreatment liquid onto an area to be recorded of the recording paper in advance of recording with an ink, an optical density of a recorded object is improved. However, in the aforementioned method, improvement of the optical density of the recorded object is insufficient.